Packages floated over the heavily militarised border by balloon also contained 1,000 United States one-dollar bills and DVDs detailing human rights abuses in the North.

"There is clearly enormous hunger for outside information in North Korea," said Thor Halvorssen, president of the US-based Human Rights Foundation, which supported the event in the border town of Paju.

"USB keys are one of the most powerful tools, because they're small, can be hidden and shared easily, and carry massive amounts of data," he said.

Each of the 1,500 USB flash drives launched yesterday had been loaded with the entire Korean-language version of the online encyclopedia Wikipedia.

The 500,000 anti-North leaflets in the packages were also accompanied by about 50 tiny transistor radios.

While North Koreans live in what is probably the most isolated and censored society on the planet, ranking last in any media freedom survey, the country is not a complete IT desert.

Cell phones were introduced through a joint venture with the Egyptian telecom firm Orascom in 2008, the same year the state launched a domestic intranet, and some government bodies have their own websites.

Yesterday's balloon launch was organised by a North Korean defectors' group.

This article appeared in the South China Morning Post print edition as S Korean activists float packages across border